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Explorer: GEOS A, B

The GEOS 1 (Geodetic Earth Orbiting Satellite)
spacecraft was a gravity-gradient-stabilized, solar-cell powered unit designed exclusively
for geodetic studies. It was the first successful active spacecraft of the National
Geodetic Satellite Program. Instrumentation included

Optical Beacon System

Laser Tracking Reflector

Radio Doppler System

SECOR Range Transponder

Radio Range/Rate System

These were designed to operate simultaneously to fulfill the objectives of locating
observation points (geodetic control stations) in a three dimensional earth center-of-mass
coordinate system within 10 m of accuracy, of defining the structure of the earth's
irregular gravitational field and refining the locations and magnitudes of the large
gravity anomalies, and of comparing results of the various systems onboard the spacecraft
to determine the most accurate and reliable system. Acquisition and recording of data were
the responsibility of the GSFC Space Tracking and Data Acquisitions Network (STADAN). Ten
major observing networks were used.

The GEOS 2 (Geodetic Earth Orbiting Satellite) was a
gravity-gradient-stabilized, solar-cell-powered spacecraft that carried electronic and
geodetic instrumentation. The geodetic instrumentation systems included

Optical Beacon System

Radio Doppler System

SECOR Range Transponder

Radio Range/Rate System

C-Band Radar Transponder

NASA Minitrack System

Precipitating Electron Detector

Magnetometer

Laser Tracking Reflector

Non-geodetic systems included a laser detector and a Minitrack interferometer beacon.
The objectives of the spacecraft were to optimize optical station visibility periods and
to provide complementary data for inclination-dependent terms established by the Explorer
29 (GEOS 1) gravimetric studies. The spacecraft was placed into a retrograde orbit to
accomplish these objectives. Operational problems occurred in the main power system,
optical beacon flash system, and the spacecraft clock, and adjustments in scheduling
resulted in nominal operations.